Actually, I think the background galaxy looks rather realistic. Our own galaxy appears to be a smear of light and dust with some stars. This is because there are so many stars so far away they blend together as seen with our eyes. You did a good job getting all the planets/moons in the same phase (i.e. illuminated from the same light source). If perfect realism is your thing, my only suggestion is that the tiny comet you have in the background isn't quite pointing towards that same light source (comet tails always point away from the star they orbit.) But perfect realism isn't necessary for great space art, either. A very nice image.